Marchand was involved in a physical altercation with Pittsburgh Penguins goalie Tristan Jarry back on February 8th. Marchand first struck Jarry in the mask with a punch and followed that up by high-sticking the netminder as the officials ushered him off the ice.
The key issue here is that Marchand will have already served 3 games of his 6-game suspension before his appeal hearing will ever have taken place at all - meaning that he will not even have had an opportunity to present his case before half of his suspension has already been served.
Everything is compounded by the fact that after the hearing, Gary Bettman will need time to deliberate before his ruling, which could also force Marchand to miss Thursday's game as well.
In addition, if Marchand's first appeal is unsuccessful, he will have the option to plead his case to an independent arbitrator, who has the power to reduce or uphold his suspension as well. If Marchand serves 4 games of his 6-game suspension even before the arbitrator has a chance to even hear his case, he may have served 5 or even all 6 games of his suspension.
In my opinion, the NHLPA should absolutely push back against the league and look to have this process amended to allow players to at least go through the appeal process before most or all of the games have been played.
One interesting thing to note is that a player can still win an appeal after serving the entire suspension. The award would simply be to recoup some of the money they had forfeited as a result of the suspension, so there's that.